Systems Biology of Organ Initiation at the Shoot Apex

The aerial tissues and organs of the plant are initiated by populations of undifferentiated, rapidly dividing cells called shoot apical meristems (SAMs). Within one species, there are different types of these meristems depending on developmental stages and environmental conditions: (1) vegetative meristems that give rise to the nonreproductive organs such as leaves or lateral branches, (2) inflorescence meristems producing flowers, and finally (3) the floral meristems producing the perianth and reproductive organs. Although these meristems produce different organs and are characterized by specific gene activities, they have a number of common structural characteristics (e.g. Lyndon, 1998; Traas and Doonan, 2001). One of these features is the presence of one or several distinct cell layers that form the so-called tunica and cover the internal tissues, collectively called corpus. Superimposed on these layers, zones can be distinguished characterized by subtle differences in cytological properties and that have particular functions. At the meristem summit the central zone (CZ) contains the true stem cells that ensure meristem maintenance. Their relatively slow proliferation rates distinguish them from the cells at the peripheral zone where the new lateral organs are initiated.